In many electronic applications, electrical resonators are required. For example, in many wireless communications devices, radio frequency (RF) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a Film Bulk Acoustic Resonator (FBAR). The FBAR has the advantage of small size and lends itself to Integrated Circuit (IC) manufacturing tools and techniques. The FBAR includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of microns and length and width dimensions of hundreds of microns, FBARs beneficially provide a comparatively compact alternative to known resonators.
Most FBAR devices have a frequency response having a band pass characteristic characterized by a center frequency. The resonant frequency depends on the materials of the FBAR ‘stack’ as well as their respective thicknesses. The constituent FBARs have a frequency response characteristic characterized by a resonant frequency. In certain known FBAR devices in which the material of the piezoelectric material is aluminum nitride (AlN) and the material of the electrodes is molybdenum (Mo), the resonant frequency of the FBAR device has a temperature coefficient ranging from approximately −20 ppm/° C. to approximately −35 ppm/° C. Such temperature coefficients reduce the temperature range over which the FBAR device incorporating the FBARs can meet its pass bandwidth specification. Such temperature coefficients additionally reduce manufacturing yield, because the bandwidth limits to which the FBAR devices are tested have to be inset to ensure that the FBAR device will meet its bandwidth specification over its entire operating temperature range.
Illustratively, the change in the temperature coefficient of the constituent materials of the FBAR device can result in a change in the resonant frequency of the FBAR device of several MHz over a typical operating temperature range of −30° C. to +85° C. As should be appreciated, variation in the resonant frequency (also referred to as the frequency shift) with temperature may be so great as to shift the operating frequency of the device outside its desired operating frequency range. For example, if the FBAR device is a component of a signal filter, a change in the resonant frequency could impact the passband of the filter beyond an acceptable limit.
In an effort to reduce the variation of resonant frequency with temperature of FBAR devices, temperature compensation layers have been developed. In certain known FBAR devices, the temperature-compensating element has a temperature coefficient opposite in sign to the temperature coefficient of the piezoelectric element. While some materials are useful in temperature compensation layers, there are drawbacks in their incorporation into fabrication of many FBAR devices.
What are needed, therefore, are an acoustic resonator structure and its method of fabrication that overcome at least the shortcomings of known described above.